The present invention relates to a switching element driving device for a power conversion device including current control semiconductor switching elements. In particular, the present invention relates to a technology of achieving an improved power conversion efficiency in such power conversion device including semiconductor switching elements.
A power conversion device including semiconductor switching elements has widely been used from the standpoint of efficient utilization of energy due to its excellent characteristics in power conversion efficiency. Example of the semiconductor switching elements include voltage-driven type elements, such as insulated-gate bipolar transistors (IGBT), static-induction type transistors, field-effect transistors (FET), and current-driven type elements, such as bipolar-mode static-induction type transistors (BSIT) and bipolar junction transistors (BJT).
The voltage driven type elements can be directly driven by voltage signals, so that their driving circuit may readily be simplified and their drive frequency may also be set higher. In applications which require 250V or more of withstand voltage, several types of switching elements are selectively used depending on the capacity and drive frequency required. Specifically, as switching elements for use in the drive frequency range of several KHz to several hundred KHz, use has widely been made of IGBTs which have an excellent overall balance between a voltage drop under ON state and a switching performance and of FETs which have a small current capacity but capable of high speed operation.
On the other hand, since the current-driven switching element is driven by applying current to its control terminal, the driving circuit often becomes complicated and the switching speed is generally lower as compared with a voltage-driven type element. However, the current-driven switching elements have a characteristic that the voltage drop under ON state is about one-third to one-sixth of the voltage-driven element. Therefore, it can be concluded that the current-driven switching element is suited to reduction in size of the power conversion device.
Thus, semiconductor switching elements usable for power conversion devices can be classified broadly into two types. Among these types, from the standpoint of size reduction of components, simplification of circuits, size reduction by high frequency driving, and cost reduction and so on, there is an increasing tendency of using voltage-driven switching elements which have a lower switching loss and a capability of being easily driven in high frequency range. However, in order to comply with social needs for achieving further improvements in efficiency and size reduction over the future, the level of voltage drop under ON state of the voltage-driven type element would be an obstacle as long as the current technologies using the voltage-driven switching elements are continued to be adopted. In particular, observing the current situation, the voltage drop under an ON state of an IGBT or the like, which is the most widely used one among voltage-driven switching elements, has already been improved closely up to a theoretical value. Thus, the technology has already been reached a highest accomplishment, so that there would be no hope in an effort to achieve a substantial reduction in the conduction loss.
As to the switching loss, developments have been made in a loss recovery technology utilizing a resonance phenomenon and a soft switching technology for the purpose of both the prevention of electromagnetic environmental pollution and the reduction of power loss. On the other hand, a conduction loss is always produced in a semiconductor switching element whenever current passes through the element and the amount of the loss depends on the characteristics of the element, so that it would not be easy to reduce the conduction loss only through a simple improvement but would require radical review of circuit topology.
In the technical field of the power conversion device, various efforts are still continued with aiming at size reduction of the device as a whole, obtaining a high power/high density device, and accomplishing a higher efficiency, and so on.
There are two primary losses produced in semiconductor switching elements of a power conversion device, one being a switching loss generated while the semiconductor switching element is moved from an ON state to an OFF state or from an OFF state to an ON state, the other being a conduction loss caused by a voltage drop produced in the semiconductor switching element when the semiconductor switching element is in the ON state. In order to comply with the requirements of making a power conversion device more compact than existing ones provide a device of higher output power and higher density, to thereby obtain a power conversion device meeting with the needs, it is necessary to develop a technology capable of accomplishing high efficiency by comprehensively reducing both the aforementioned conduction loss caused by the voltage drop under ON state of the semiconductor switching element and the switching loss which together lead to a power loss.
Under the above circumstances, there are very few examples reporting that the conduction loss in a semiconductor switching element has been reduced through an effective improvement in circuitry. Referring to examples among these reports, Japanese Patent Laid-Open Publication No. Hei 1-97173 discloses a technology for reducing both switching loss and conduction loss in a PWM full bridge power conversion device such as a PWM inverter by providing a semiconductor switching element having small conduction loss, such as a bipolar transistor, in an arm adapted to be switched under a commercial-frequency, and a semiconductor switching element having small switching loss, such as static-induction transistor, in an arm adapted to be switched under a high-frequency. The Transactions of the Institute of Electrical Engineering of Japan (T. IEE Japan), Vol. 116D, No12 (1996), also discloses a circuitry improvement for reducing conduction losses in a power conversion device using semiconductor switching elements. However, these prior art technologies still include problems in that adequate investigations have not been made in respect of optimization of conduction loss, reduction of loss in the driving circuit and size reduction, and that there is practically limitations in the driving frequency. For example, the aforementioned Japanese Laid-Open Publication includes no specific teaching about method for driving the bipolar transistor which is of a current control type switching element. However, when a constant current is applied to the base as in a conventional method for driving a transistor, the efficiency under a light load condition would particularly become low due to the driving loss in no load condition or light load condition. In the technology described in the aforementioned Transactions of the Institute of Electrical Engineers of Japan, drive power is supplied to a transistor by a CT (current transformer) which is connected to the collector of the transistor, so that the base current is determined by the winding ratio of the aforementioned CT. Therefore, it is required that the circuit is to be designed in consideration of the minimum value of current amplification factor of the semiconductor switching element. As the result, there is a possibility that the CT is driven up to a supersaturated state in low load. Besides, the technology is effective only under a relatively high frequency due the use of the CT.
Under these circumstances, it is an object of the present invention to provide a power conversion device including a semiconductor switching element and capable of achieving high efficiency by totally reducing the switching loss and the conduction loss produced at the switching element.
In order to accomplish the above and other objects, the present invention proposes to detect the collector current or drain current in the semiconductor switching element of the power conversion device, and control the base current to reduce the sum of the conduction loss and the drive power in the aforementioned switching element in accordance with the detected value.
More particularly, according to the present invention, there is provided a switching element driving device for a power conversion device including a current control type semiconductor switching element which has a collector or drain, an emitter or source, and a base or gate. The driving device comprises an output main line connected to the base or gate of the switching element, an output return line connected to the emitter or source, a collector current detecting device for detecting a collector current or drain current, and a base current control device for controlling a base current or gate current. The base current control device controls the base current to reduce the sum of conduction loss and drive power in the switching element in accordance with a collector current signal from the collector current detecting device.
In one aspect of the present invention, the driving device further comprises an element-drive power supply for providing a current for driving the switching element, and a reverse bias power supply for applying a reverse bias to the base or gate of the switching element. The base current control device includes an ON-drive switch device for connecting the element-drive power supply to the base or gate of the switching element, an OFF-drive switch device for connecting the reverse bias power supply to the base or gate of the switching element, an operation device for receiving the collector current signal from the collector current detecting device and performing an arithmetical operation to obtain the base current. Further, the ON-drive switch device is held in a conducting state and the OFF-drive switch device is held in a non-conductive state when the switching element is to be turned on. The OFF-drive switch device is then held in a conducting state and the ON-drive switch device is held in a non-conductive state when the switching element is to be turned off, so that the reverse bias from the reverse bias power supply is applied for a rapid OFF operation of the switching element.
According to another aspect of the present invention, the base current control device stores a data of optimum base current with respect to the collector current which is specific to the particular switching element, and includes a base current determining device for determining the base current value, on the basis of the stored data, in accordance with the collector current signal from the collector current detecting device, and a current control device for applying the base current to the base or gate of the switching element in accordance with a signal from the base current determining device.
In a specific aspect of the present invention, the base current control device stores the data of optimum base current corresponding to various different temperatures. Further, there is provided a temperature detecting device for detecting a temperature of the switching element, the determining device of the base current control device being adapted to determine the base current value, on the basis of the stored data, in accordance with to the collector current signal from the collector current detecting device and a temperature signal from the temperature detecting device.
The determining device may be configured to perform an arithmetical operation to obtain the base current which minimizes the sum of the conduction loss and the drive power of the switching element and then to multiply the value thus obtained by a coefficient greater than 1 to provide an output to the current control device. The determining device may also be configured to perform an arithmetical operation to obtain the base current which minimizes the sum of the conduction loss and the drive power of the switching element and then to add a constant current value to the value thus obtained to provide an output to the current control device. The determining device may, alternatively, be configured to perform an arithmetical operation to obtain the base current which minimizes the sum of the conduction loss and the drive power of the switching element and then to multiply the value thus obtained by a coefficient greater than 1 in conjunction with adding a constant current value thereto to provide an output to the current control device. The current control device may include a switch device for controlling the base current of the aforementioned switching element and a rectifying device provided in the output portion of the switch device. The rectifying device may be of a synchronous type including a rectifying element and an auxiliary rectifying element which has a lower conduction resistance than the switch device. A base current detecting device for detecting the base current of the switching element may further be provided and this base current detecting device may be configured to regulate the base current signal from the base current control device in accordance with a base current command signal which is obtained in accordance with the collector current signal from the aforementioned collector current detecting device.